In contrast to earlier reports, it is shown that dialkyl nitroxides are stable in strong acid solutions at room temperature. The nitroxides yield epr spectra indicating that they are protonated in solutions such as 100% H2SO4 which have Hammett acidity function values, H0, more negative than ca. -7.5. In solutions having H0 more positive than ca. -3.5 the unprotonated nitroxide epr spectrum is obtained. Between these two acidities no epr signal is observed because of line broadening resulting from rapid proton exchange. The pKa for dialkyl nitroxides is -5.5 ± 1. It is concluded from the 14N hyperfine coupling constants that the geometry of the nitrogen atom in a particular protonated nitroxide is very similar to that in the same unprotonated radical despite the fact that the absolute geometry for different dialkyl nitroxides varies substantially. The value of the proton hyperfine coupling constant decreases as the angle that the NO bond makes with the CNC plane increases.